Development of a Mixture Design Procedure for Stone Matrix Asphalt

Development of a Mixture Design Procedure for Stone Matrix Asphalt

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    Stone Matrix Asphalt (SMA) has been used successfully in Europe for over 20 years to provide better rutting resistance and to resist studded tire wear. Since 1991, the use of SMA has increased steadily in the United States. At present, some states routinely use SMA even though a standard mixture design procedure is not available. A mixture design procedure that provides guidance on material properties, aggregate gradation, determination of optimum asphalt content, and mixture properties is needed. This paper presents a mixture design procedure for SMA mixtures developed by the National Center for Asphalt Technology. Data for the development of the procedure was collected from a laboratory study conducted with various samples of aggregates, fillers, asphalt binders, and stabilizing additives. Compacted mixtures were tested to evaluate the effects of aggregate structure, asphalt binder, and binder-fine aggregate mortar. Specific conclusions from this study were: (1) The Los Angeles abrasion loss showed good correlation with aggregate breakdown, (2) It appeared that the 3:1 or 2:1 flat and elongated particles provided much better classification for the various aggregates than a 5:1 ratio, (3) The flat and elongated particle ratio showed excellent correlation with aggregate breakdown, (4) In a SMA mix, the percent passing the 4.75 mm sieve must be below 30 percent to ensure proper stone-on-stone contact, (5) The percent passing the 0.02 mm sieve did not show a correlation with mortar stiffness. However, the dry compacted volume, as obtained from the Penn State test method, did show a good correlation with mortar stiffness and can be utilized to characterize the shape of fillers. Generally, a more angular filler tends to produce a higher air voids result in this test, (6) In-place results from about 86 projects showed that very little rutting has occurred in SMA pavements constructed in the United States since 1991. However, for the pavements with air voids falling below the 3 percent range, some rutting was observed. (7) A VMA significantly lower than specified VMA can be obtained due to aggregate breakdown. Hence, the mix designer must consider aggregate type, compactor type and compactive effort along with the gradation in meeting the required VMA criteria. Specifying a minimum asphalt content can result in different requirements for aggregates with different specific gravity, (8) Fifty blows of Marshall hammer were found to be approximately equal to 100 revolutions of the Superpave gyratory compactor in terms of resultant density. The Superpave gyratory compacter was found to produce less aggregate breakdown than the Marshall hammer, (9) Fiber stabilizers were found to be more effective in reducing draindown than polymer stabilizers. However, mixes modified with polymer showed better resistance to rutting in laboratory wheel tracking tests.
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